Technology-based remote research methods are increasingly widespread, including learning assessments in child development and education research. However, little is known about whether technology-based remote assessments remain as valid and reliable as in-person assessments. We developed a low-cost phone-based language and literacy assessment for primary-school children in low-resource communities in rural Côte d'Ivoire using voice calls and SMS. We compared the reliability and validity of this phone-based assessment to an established in-person assessment. A total of 685 5th grade children completed language (phonological awareness, vocabulary, language comprehension) and literacy (letter, word, pseudoword, passage reading, and comprehension) tasks in-person and by phone. Reliability (internal consistency) and predictive validity were high across in-person and phone-based tasks. Children's performance across in-person and phone-based assessments was moderately to strongly correlated. Phonological awareness and vocabulary skills measured in-person and by phone significantly predicted in-person and phone-based letter, word, and pseudoword reading. Oral language and decoding skills measured in-person and by phone significantly predicted in-person and phone-based passage reading and comprehension. Our phone-based assessment was a reliable and valid measure of language and reading and feasible for low-resource settings. Low-cost technologies offer significant potential to measure children's learning remotely, increasing the inclusion of remote and low-resource populations in education research.